Insulitis, inflammation due to infiltration of immune cells in and around pancreatic islets, is a hallmark of type I diabetes. Different stages of insulitis have been observed in non-obese diabetic (NOD) mice and BioBreeding (BB) rats. Mechanisms involved in the destructive progression of insulitis, however, are not clearly understood. In this study, we examined the role of the cytokine interluekin-1 (IL-1) in stimulating islet cells to produce a chemotactic factor(s) that may play a role in the progression of insulitis. IL-1 induces accumulation of non-=esterified arachidonic acid in islets as demonstrated by isotope dilution mass spectrometry. Some arachidonate metabolites have chemotactic activity. We have observed that conditioned media collected from rat islets treated with 5 units/ml IL-1 contained chemotactic activities for human monocytes determined by modified Boyden chamber assays. The production of chemotactic activity by islets exposed to IL-1 was time-dep endent and completely blocked by treating islets with 1(M actinomycin D. Addition of antiserum raised against rat macrophage chemoattractant protein-1 (MCP-1) to conditioned media significantly inhibited monocyte chemotaxis, suggesting that MCP-1 may, in part, be responsible for the chemotactic activity observed. We confirmed the synthesis of MCP-1 mRNA by rat islets using in situ hybridization. (-cells of the istlet appear to be the source of MCP-1 since primary (-cells and the insulinoma cell line, RINm5F, express MCP-1 mRNA upon IL-1 activation. These results indicate that pancreatic (-cells produce MCP-1 upon IL-1 activation that may play a role in the progression of insulitis associated with autoimmune diabetes.